Central kappa opioid agonists and nociceptin (a newly identified opioid-like peptide) evoke profound changes in urine flow rate (diuresis) and urinary sodium excretion (antinatriuresis). The selective water diuresis that each of these opioids produce is unique and is likely to play an important physiological role in maintaining homeostasis of body fluid and electrolyte balance under states in which fluid overload and/or sodium deficiency may provoke hyponatremia/hypoosmolality. While central kappa opioids and nociceptin produce a similar pattern of changes in renal excretory function, there remain important unanswered questions regarding the central sites and mechanisms at each locus by which opioid systems control the renal handling of water and sodium and the functional importance of these regulatory systems. The studies outlined in this amended proposal will test the Overall Hypothesis that central kappa opioids and nociceptin act within discrete brain nuclei to influence neural and humoral pathways that control water and sodium excretion under different physiological conditions. These studies will use methods for measurement of changes in cardiovascular and renal function in conscious rodents and mice. The Specific Aims are: 1) To examine the hypothesis that the paraventricular nucleus of the hypothalamus (PVN) and the rostral ventrolateral medulla (RVLM) are brain sites where kappa opioids and nociceptin act to produce changes in cardiovascular and renal function and renal sympathetic nerve activity in conscious Sprague-Dawley rats; 2) To examine the hypothesis that angiotensin II contributes to the cardiovascular and renal responses produced by central nociceptin in conscious Sprague-Dawley rats; and 3) To examine the hypothesis that endogenous central kappa opioid and nociceptin systems are important in the physiological control of cardiovascular and renal function during acute environmental stress and long-term cardiovascular and fluid regulation. Note that studies in Specific Aim 2 are directed toward central nociceptin since we have already established the neural and humoral pathways by which central kappa opioids affect the renal excretion of water and sodium. The unique aspects of this amended proposal are that we will use a newly identified and selective ORL1 receptor antagonist, UFP-IO1, and transgenic ORL1 receptor knockout mice as two effective and innovative approaches to investigate the physiological importance of central endogenous kappa opioids and nociceptin in the acute/long-term regulation of cardiovascular and fluid regulation. Together, the findings of these studies will provide new and important information regarding integrated cardiovascular and renal mechanisms involved in the central control of blood pressure and fluid/electrolyte homeostasis.